An information recording medium including a CD, a DVD, and the like has become popular. Such an information recording medium has a recording film, such as an organic dye film and a phase change film, formed on a substrate by spin coating or the like, and a light transmission film with a thickness of 0.6 mm to 1.2 mm, formed on the recording film by spin coating or the like.
In recording data onto the information recording medium, tracking control is performed in order to irradiate laser light at a preferable position. The tracking control is performed to irradiate the laser light near the center of a recording track. More specifically, the tracking control is performed on the basis of a push-pull signal, which indicates a difference between a right-side signal component and a left-side signal component of reflected light of the laser light, with respect to a recording direction.
The tracking control needs to be performed preferably when a disturbance, such as an impact, is exerted on an information recording apparatus and even when there is a defect, such as a scratch and dusts, on the surface of the information recording medium. For example, when the disturbance is exerted, a tracking error signal used for the tracking control is significantly deteriorated, so the tracking control is performed according to the deterioration (in other words, so as to eliminate the deterioration). When there is the defect, although the laser light is irradiated near the center of the recording track, the tracking error signal is significantly deteriorated. Thus, with reference to a defect detection signal generated on the basis of a sum signal (or a RF signal), the tracking control is performed to hold tracking servo. As a method of performing the tracking control when there is such a disturbance and a defect, for example, methods in patent documents 1 and 2 are listed.
Moreover, in addition to the disturbance and the defect, an air bubble with a size of about several tens micrometers (hereinafter referred to as an “air bubble”) is generated in some cases in the light transmission film, depending on conditions in a manufacturing process. Even if the air bubble is generated, the aforementioned tracking control needs to be preferably performed.    Patent document 1: Japanese Patent Application Laid Open NO. Hei 11-39678    Patent document 2: Japanese Patent Application Laid Open NO. Hei 8-147721